Due to the recent progress in the art of photosensitive materials for photograph-taking use, newly developed photosensitive materials of high photographic speed have been highly commercialized. The expansion of the photographing environment depends on the attainment of high photographic speed in photosensitive materials, for instance, photographing in a dark room without a strobe light, photographing of, e.g., sports scenes, through a telephoto lens while rapidly handling the shutter, photographing requiring many hours of exposure, e.g., taking astrophotographs, and so on.
For the purpose of increasing the photographic speed of a photosensitive material, considerable efforts have been expended. A great number of methods for forming silver halide grains having a desired form and composition, chemical sensitization, spectral sensitization, additives, coupler structures, and so on have been developed. One method involves combining a method of enlarging the size of the silver halide grains with another method of increasing the photographic speed. This method has been a typical measure for producing a photosensitive material of high photographic speed in the photographic arts. However, the progress of the art of photography is still behind the requirements for photosensitive materials of a high photographic speed.
More specifically, although enlargement of the size of the silver halide emulsion grains can increase the photographic speed to some extent, it necessarily leads to a decrease in the number of silver halide emulsion grains, provided that the content of silver halide in the emulsion is maintained constant. As a results, the number of development initiation centers is decreased. Therefore, the increase in size of the silver halide grains entails a disadvantage in that the graininess is greatly spoiled. In order to offset this disadvantage, various methods have been proposed. For instance, a method of using a photosensitive material containing at least two emulsion layers which has the same color sensitivity, but different photographic speeds, that is, different grain sizes, respectively, as described in British Patent 923,045 and Japanese Patent Publication No. 15495/74; a method of using a rapidly reacting coupler, as described in Japanese Patent Application (OPI) No. 62454/80 (the term "OPI" as used herein means an "unexamined published application"); a method of using a so-called DIR coupler or DIR compound, as described in U.S. Pat. Nos. 3,227,554 and 3,632,435; a method of using a coupler capable of producing a diffusible dye, as described in British Patent 2,083,640; a method of using silver halide grains having a high mean silver iodids content, as described in Japanese Patent Application (OPI) No. 128443/85; and so on are well-known. Although these methods each has a great effect and can be said to be an excellent invention, they are still insufficient to meet many of the requirements for heightening both the photographic speed and the image quality. Therefore, in order to increase the grain size of the silver halide emulsion and at the same time, to increase the number of development initiation centers to as large as possible, high-speed color negative photosensitive materials have been designed to contain silver halide emulsion grains in the largest amount as possible so that various properties, such as the desilvering capacity at the time of bleach-fix processing, are not adversely effected. However, the thus produced photosensitive materials having a high photographic speed and a high image quality have turned out to suffer from the following undesirable disadvantages.
A first disadvantage is the deterioration of photographic properties, including an increase in fog, a decrease in the photographic speed, a deterioration in the graininess, and so on, which occur during the preservation period, i.e. between the production of the photosensitive material and the use thereof. In particular, deterioration in the graininess is a serious problem. It has been found in our investigations that the main cause of the deterioration is that the light-sensitive silver halide emulsion grains are exposed to natural radiations, such as Gammarays, emitted from building materials, the ground, etc., cosmic rays, and so on. Although the properties of a photosensitive material were already known to be deteriorated by irradiation with X-rays or high energy radiations, it was newly found that with regard to a high-speed color negative photosensitive material having a specific photographic sensitivity of 800 or above, the definition of which is described hereinafter, the photographic properties thereof are deteriorated greatly beyond anticipation even by exposure to very weak natural radiation.
In order to prevent the deterioration of the above-described kind, a method of shielding radiations by using a material having a high absorption coefficient with respect to radiations, such as lead, for a packing material or as a material for making a preservatory, as described, e.g., in Research Disclosure, No. 25610 (August 1985) can be employed. However, it is necessary to use a heavy metal, like lead, in a considerable thickness in order to carry this method into perfection. If the thickness is insufficient, the above-described aim cannot be achieved. Accordingly, it is nearly impossible to supply such a material as described above to the consuming public with ease at a low price.
A second disadvantage is the impossibility of fulfilling the severe requirements for the image quality of the photosensitive materials recently developed.
A third disadvantage is the inferiority of the photosensitive material in terms of pressure resistance, and more specifically in terms of generating abrasive fog (sensitization) upon the speeding-up of processing, and upon the use of cameras fitted with a high speed automatic rolling-up mechanism.
In addition, a fourth disadvantage is poor fixation or poor desilvering in the conventional photosensitive material of the high-speed type upon a decrease in the development processing time and a decrease in the amount of replenisher added to the processing solution.